It is well known in the art to use glass fibers as reinforcements for polymeric materials such as composites formed by sheet molding compound (SMC) and bulk molding compound (BMC) processes. Such composites are formed using glass fiber reinforcements which provide dimensional stability and excellent mechanical properties to the resulting composites. For example, glass fibers provide dimensional stability as they do not shrink or stretch in response to changes in atmospheric conditions. Further, glass fibers have high tensile strength, heat resistance, moisture resistance, and high thermal conductivity.
Glass fibers are commonly manufactured by supplying ceramic in molten form to a bushing, drawing fibers from the bushing, and then gathering the fibers into a tow or strand. A sizing composition, or chemical treatment, is typically applied to the fibers after they are drawn from the bushing, which protects the fibers from breakage during subsequent processing and improves the compatibility of the fibers with the matrix resins that are to be reinforced. The sized strands are typically wound onto a collet, packaged, dried, and then wound together into a continuous roving. An antistatic overcoating is applied to the roving during the winding step. The roving is then subsequently chopped and contacted with a matrix resin in the sheet molding or bulk molding compound to form molded composites.
Typical sizing compositions may include coupling agents, film formers, lubricants, emulsifiers, or antistatic agents that are dissolved or dispersed (in the form of an emulsion or dispersion) in water. However, some organic solvents commonly used to emulsify or disperse film formers used in SMC size compositions, such as styrene and xylene, are flammable and pose a fire and health hazard. Lithium chloride is also commonly used in sizing compositions as an antistatic agent, but tends to adversely affect yield and is therefore undesirable for use. Accordingly, it would be desirable to use a sizing composition which does not utilize undesirable organic solvents or metal halide salts.
It would also be desirable to use a size composition which provides a smooth surface to the resulting molded part. For example, Das et al., in U.S. Pat. No. 4,338,233, describe aqueous sizing compositions for glass fibers which provide a smooth surface to molded compounds. However, such sizing compositions have a low solubility as they comprise one or more crosslinkable film forming polymers which, when crosslinked, become insoluble. Such low solubility size compositions are desirable for use in automotive or Class A applications where the resulting composite parts are painted. However, a low solubility size composition is undesirable for applications where the fibers are utilized in non-pigmented or light-pigmented sheet molded parts which are not subsequently painted because the fiber strands remain integral in the molding compound, i.e., the fibers do not sufficiently defilamentize. Thus, the fiber network is visible within the resulting molded part.
In the production of structural, low-pigmented or non-pigmented parts, the sizing composition should be highly soluble so that the individual fibers are sufficiently dispersed or wetted by the matrix resin. This promotes better fiber strand defilamentization, or strand breakup, which reduces fiber prominence and thus improves the uniformity or smooth appearance of the surface of the resulting composite. This also promotes an increased interface between the individual fibers and the matrix resin and results in better mechanical properties which are needed in structural applications.
Accordingly, a need still exists in the art for an improved size composition which exhibits high solubility, and is easy to manufacture and apply to fibers. There is a further need in the art for a size composition which improves the surface appearance of non-pigmented or low-pigmented parts formed by sheet molding and bulk molding processes, and which does not use include environmentally undesirable solvents or metal halide salts.